For battery separators, non-woven fabrics comprising nylon and polypropylene fibers are generally used. Some non-woven fabrics called dry laid type non-woven fabrics are manufactured by a dry laid process, while other non-woven fabrics manufactured by a wet laid process are called wet laid type non-woven fabrics. As non-woven fabrics comprising nylon fibers have poor alkaline resistance, non-woven fabrics comprising polyolefin-based fibers such as polypropylene are preferably used.
Such non-woven fabrics comprising polyolefin-based fibers are, however, hydrophobic, and thus, they have poor wettability when used for battery separators. Therefore, various methods for providing hydrophilicity to polyolefin-based non-woven fabrics have been proposed. One of the well-known examples is a treatment of applying hydrophilic surfactants to non-woven fabrics for hydrophilicity. Tokko-Hei (Published Examined Japanese Patent Application) No. 1-36231 discloses a non-woven fabric comprising sheath-core type bi-component fibers of polypropylene/polyethylene with which vinyl monomer is graft-copolymerized. Tokko-Hei 5-46056 discloses a polypropylene non-woven fabric with which fluorine gas is contact-reacted, and Tokkai-Hei (Published Unexamined Japanese patent Application) No. 7-142047 discloses that splittable bi-component fibers comprising polyolefin/ethylene vinyl alcohol copolymer and polyolefin-based fibers are mixed and processed, entangled by using high-pressure water flow, and treated with corona surface discharge.
The above-mentioned battery separators, however, have several problems. For instance, a battery separator manufactured by providing a hydrophilic surfactant to a non-woven fabric for hydrophilicity is superior in the initial alkaline absorption and alkaline retaining property, but the surfactant adhered to the surface of the non-woven fabric will be washed away because of the repeated charge and discharge of the battery. As a result, the wettability of the alkaline electrolyte will be greatly lowered, and the battery life will be shortened.
The battery separators disclosed in Tokko-Hei 1-36231 and Tokko-Hei 5-46056 have non-woven fabrics having surfaces modified to provide hydrophilicity in order to improve durable hydrophilicity. These separators, however, require special work and the processability and productivity are poor, resulting in increased cost. Therefore, the separators are not practical.
Tokkai-Hei 7-142047 discloses a battery separator comprising 75-100 weight % of splittable bi-component fibers comprising hydrophobic polyolefin polymer and hydrophilic ethylene vinyl alcohol copolymer. Although the durable hydrophilicity of the fibers is improved by treating with corona charge for providing hydrophilic groups, the air permeability of the non-woven fabric is low due to the excessively small fiber gaps, and thus, the gas permeability required for a sealed battery is inferior.